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Al2O3 spherical catalysts
Applied Catalysis A: General 306 (2006) 134–141 www.elsevier.com/locate/apcata
Production of hydrogen by steam reforming of ethanol over Ni/Al2O3 spherical catalysts Humberto Vieira Fajardo *, Luiz Fernando Dias Probst Laborato´rio de Cata´lise Heterogeˆnea, Departamento de Quı´mica, Universidade Federal de Santa Catarina, 88040-900 Floriano´polis, SC, Brazil Received 25 November 2005; received in revised form 17 March 2006; accepted 20 March 2006 Available online 27 April 2006
Abstract A new and simple method to prepare Al2O3 and Ni/Al2O3 spherical catalysts for hydrogen production by steam reforming of ethanol is proposed. The present method was developed using a biopolymer (chitosan) and Al solution. A hybrid spherical compound of aluminum hydroxide and the biopolymer is formed. Through polymer elimination by thermal treatment, porous spheres with a high specific surface area and large pore volume are obtained. The Ni/Al2O3 spheres showed a high catalytic activity in ethanol steam reforming and hydrogen selectivity at temperatures between 450 and 650 8C. In all cases no CO was detected indicating that the Ni/Al2O3 spheres could be considered as a good choice for hydrogen production from ethanol steam reforming for fuel-cell applications. # 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Alumina; Catalysts; Steam reforming
1. Introduction Hydrogen is nowadays considered as an alternative fuel, and its production is a subject of current interest for fuel cell applications. Fuel cells are considered to have the potential to provide a clean energy source for automotive applications as an alternative to gasoline or diesel engines [1–4]. In recent years, the proton exchange membrane fuel cell (PEMFC) with hydrogen as the fuel has attracted great interest due to its potential application in electric vehicles and power stations. However, PEMFC uses pure H2 or H2-rich gas as a fuel, which needs to be stored or produced on-board a vehicle. Due to the lack of hydrogen storage and distribution infrastructure, the catalytic on-board production of H2-rich gas from a suitable fuel is attracting increasing attention [5–8]. Conventional methods for hydrogen production are based on gasoline, natural gas and methanol steam reforming [9]. However, although several sources of hydrogen have been investigated alcohols,
* Corresponding author at: Universidade Federal de Santa Catarina, Centro de Cieˆncias Fı´sicas e Matema´ticas, Departamento de Quı´mica, Po´s-Graduac¸a˜o em Quı´mica, Campus Universita´rio, CP 476, CEP 88040-900 Trindade, Floriano´polis, SC, Brazil. Tel.: +55 48 3331 9966; fax: +55 48 3331 9711. E-mail address: [email protected] (H.V. Fajardo). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.03.043
and in particular ethanol, which is a renewable material easily obtained from biomass, might be considered as a good choice, especially for Latin American countries with extensive plantations of sugar cane [9–11]. Ethanol has relatively high hydrogen content, and its reaction with water under steam reforming conditions: C2 H5 OH þ 3H2 O ! 2CO2 þ 6H2 ;
(1)
DH298 ¼ þ347:4 kJ=mol has been shown to be entirely feasible from a thermodynamic point of view [11–14]. In addition, the bio-ethanol-to-hydrogen system has the significant advantage of being nearly CO2 neutral, since the carbon dioxide produced is consumed for biomass growth, thus offering a nearly closed carbon loop [7,15]. In previous studies, several catalysts have been proposed to be further considered for practical applications in ethanol steam reforming for hydrogen production for fuel cell applications. Although many noble metals (Rh/Al2O3, Pt/Al2O3, Pd/ Al2O3, Ru/TiO2, Rh/CeO2–ZrO2) [3,16–19] and non-noble metals (Ni/Al2O3, Cu/Al2O3, Fe/Al2O3, Ni/La2O3, Ni/ La2O3–Al2O3, Ni/Y2O3, Co/Al2O3, Co/SiO2) [15,19–22] on different supports have been shown to be active in this process, Ni seems to be the preferred active ingredient because of its high activity and low cost.
H.V. Fajardo, L.F.D. Probst / Applied Catalysis A: General 306 (2006) 134–141
In this context the objective of this study was to develop a new, effective and highly efficient Ni-based catalyst for the steam reforming of ethanol to produce hydrogen with high selectivity. The synthesis method for the novel catalysts (Al2O3 and Ni-doped Al2O3) consists of obtaining a hybrid spherical compound of aluminum hydroxide and the organic polymer chitosan. Through the polymer elimination by thermal treatment, a porous Al2O3 sphere is obtained. The catalytic performances of the samples obtained were investigated in ethanol steam reforming. The key aim of this process is to maximize H2 production, discouraging at the same time reactions that lead to undesirable products, such as methane, acetaldehyde, diethyl ether or acetic acid, that compete with H2 for the hydrogen atoms, and minimize the formation of carbon monoxide, which acts as a poison to the fuel cell electrodes. In addition, the formation of large amounts of CO would require a complex gas clean up process downstream before feeding into the fuel cell [7]. The results of the catalytic behaviour of the Ni/ Al2O3 spheres in the steam reforming of ethanol for hydrogen production are reported here. 2. Experimental 2.1. Sample preparation and characterization For the Al2O3 spheres preparation, 1.50 g of chitosan (Aldrich) was dissolved in 50 mL of CH3COOH solution (5%, v/v) and 4.60 g of Al(NO3)39H2O (Riedel-de-Hae¨n) were dissolved in 20 mL of distilled water. The Al aqueous solution was then added to the polymer solution with stirring. The chitosan monomer to Al molar ratio was 1.5–2. The Al– chitosan solution was added into a NH4OH solution (50%, v/v) under vigorous stirring, in the form of drops with a syringe pump. The gel spheres formed were removed from the NH4OH solution and dried at ambient temperature for 72 h. The nickel addition (5 wt.%) was carried out by impregnation of an aqueous solution of Ni(NO3)26H2O (Fluka, 98%) on the Al2O3 spheres with stirring for 6 h. The samples with nickel were dried at ambient temperature for 24 h. The Al2O3 (Al2O3#350, Al2O3#550 and Al2O3#700) and Ni/Al2O3 (Ni/Al2O3#550 and Ni/Al2O3#700) samples were obtained by calcining the dried samples at 350, 550 and 700 8C in airflow for 1 h with a heating rate of 5 8C/min. The precursors were characterized by N2 adsorption/ desorption isotherms obtained at the temperature of liquid nitrogen in an automated physisorption instrument (Autosorb1C, Quantachrome Instruments). Prior to the measurements, the samples were outgassed in a vacuum at 200 8C for 2 h. Specific surface areas were calculated according to the Brunauer– Emmett–Teller (BET) method, and the pore size distributions were obtained according to the Barret–Joyner–Halenda (BJH) method from the adsorption data. Infrared patterns were obtained from 400 to 4000 cm1; the samples (chitosan and Al–chitosan dried samples, 2 mg) were mechanically blended with 200 mg of KBr. The data obtained were recorded using an FT Perkin-Elmer 16 PC infrared spectrophotometer.
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To monitor the pyrolysis process, a thermogravimetric analysis (TG) was performed with a Shimadzu TGA-50 thermobalance using 15 mg of sample with a heating rate of 10 8C/min under airflow of 50 mL/min. For the determination of the Ni content an atomic spectrometer (Varian Model SpectrAA 50), equipped with an air–acetylene flame atomizer and a Hitachi hollow cathode lamp (HLA—4S) was used. The instrument parameters were: wavelength 232 nm, slit width 1.0 nm, lamp current 4 mA, aspiration rate 5 mL/min and fuel acetylene with support air. The standard solutions for the Ni ion were prepared through appropriate dilution of a stock solution (Merck) containing 1000 ppm. Temperature programmed reduction (TPR) analysis was performed in a quartz reactor under 5 vol.% H2/N2 flow (30 mL/min) from 30 to 920 8C at a heating rate of 5 8C/min. A thermal conductivity detector (TCD) was used to monitor the H2 consumption. A PM5A column trapped water formed during the process. The crystalline phases (of freshly prepared catalysts and their reduced forms) were characterized by X-ray diffraction (XRD) in a Siemens D-5000, with graphite monochromated Cu Ka irradiation. The sample morphology was observed with scanning electron micrographs, obtained with a Philips XL30 scanning microscope operating at an accelerating voltage of 20 kV. The structures of the deposited carbon were characterized by means of Raman spectroscopy (Renishow Raman System 3000) at room temperature. Spectra were recorded using the 514.5 nm excitation line of an Ar ion laser. 2.2. Catalytic testing Catalytic performance tests were conducted at atmospheric pressure with a quartz fixed-bed reactor (inner diameter 12 mm) fitted in a programmable oven, in the temperature range of 450–650 8C. The catalyst was previously reduced in situ with hydrogen at 650 8C for 1 h. The water–ethanol mixture (molar ratio 3:1) was pumped into a heated chamber and vaporized. The water–ethanol gas (N2) stream (30 mL/ min) was then fed to the reactor containing 100 mg of catalyst. The reactants and the composition of the reactor effluent were analyzed with a gas chromatograph (Shimadzu GC 8A), equipped with a thermal conductivity detector (TCD), Porapak-Q and a 5A molecular sieve column with Ar as the carrier gas. It should be noted that no condensable products (such as acetic acid, diethyl ether and acetone) were detected in the reaction effluent under the analysis conditions employed. Reaction data were recorded for 5 h. Catalyst activity was evaluated in terms of ethanol conversion. We defined ethanol conversion as: CEtOH ð%Þ ¼
Qconv 100: QEtOH
(2)
Here, Qconv represents the quantity (moles) of converted ethanol; QEtOH represents the total quantity (moles) of ethanol fed into the reactor.
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We defined the catalyst selectivity as the mole fraction of each product as: SP ð%Þ ¼
QP 100: QsP
(3)
Here, QP represents the number of moles of each product; QsP represents the sum of the moles of the products, but the moles of solid products (such as small amounts of coke) are not included. It should be noted that the catalysts tested did not show deactivation during the reaction period (5 h of operation). 3. Results and discussion 3.1. Sample characterization One of the main applications of infrared spectra is in the identification of characteristic functional groups, where relatively large molecules are involved. The infrared spectra for chitosan (Cht) and the Al–chitosan composite (Al–Cht) taken before the calcination processes (Fig. 1) were analyzed to obtain information about the functional groups that participate in the binding or interaction with Al in the intermediate stage of the porous Al2O3 spheres synthesis. The hydroxyl groups of the free alcohols, thus not associated, have O–H stretching in the form of a narrow band at 3620 cm1. In polymeric associations hydroxyl groups absorb in the form of a broad band at around 3400 cm1 [23]. The bands in the region of 3440 cm1 in the two spectra are associated with stretching of the OH groups of the chitosan biopolymer. There was no displacement in relation to the wavenumber indicating that the intermolecular interactions by way of hydrogen bonds of the biopolymer were maintained. The decrease in the intensity of the band on the spectrum for the compound (Al–chitosan) is due to the interaction of Al with the oxygen atoms of the hydroxyl groups of the biopolymer bound to the glycoside ring. The Al ions are preferentially bound to oxygen. The disappearance of the band at around 1650 cm1 is related to the interaction of Al with carbonyl groups from the partially deacetylated chitin residues.
Fig. 1. Infrared spectra of chitosan (Cht) and chitosan–Al (Al–Cht) composite before the heat-treatment process. Inset: chemical structure of chitosan.
Fig. 2. Thermogravimetric analysis of chitosan–Ni–Al (Cht–Ni–Al) and chitosan–Al (Cht–Al).
In the region around 1100 cm1 there was a decrease in the characteristic stretching of the C–O groups of the carbon bound to the OH group (primary alcohol) of chitosan due to the interaction with Al. The infrared spectra therefore indicate a modification in certain characteristic regions of the functional groups of the biopolymer susceptible to interaction with Al, without there being a significant modification in the semi crystalline chitosan structure. The thermogravimetric analysis for the samples presented in Fig. 2, shows the elimination of residue from the starting material, which depends on the sample composition (chitosan– Al and chitosan–Ni–Al). The nickel presence promotes faster carbon elimination during the heat-treatment. The residue elimination is observed in different temperature ranges with a substantial weight loss, near 75 wt.% for both samples. A clear inflexion point near 250 8C is mainly attributed to the external organic material elimination, which is easily burned off together with the residual nitrate. The chitosan–Ni–Al (Cht– Ni–Al) exhibited a second inflexion point located at 380 8C, while for chitosan–Al (Cht–Al) this second inflexion point was at 310 8C with another at 530 8C, being attributed to the residual carbon present within the spheres. This distinct behaviour is due to the presence of nickel. It is known that a metal (Ni) load in the sample promotes the elimination of carbonaceous materials at a lower temperature [24–26]. The TGA profile suggests a temperature of 400 and 600 8C for the total elimination of the residual material with a short calcination time for the chitosan–Ni–Al and chitosan–Al samples, respectively. The samples were heat-treated at 350, 550 and 700 8C under airflow for 1 h. The N2 adsorption/desorption isotherms of the Al2O3 samples heat-treated at 550 and 700 8C show type-IV (IUPAC) curves, pointing to the mesoporous material. By contrast, the Al2O3 sample heat-treated at 350 8C shows a microporous isotherm (Fig. 3a). The N2 adsorption/desorption isotherms of the Ni/Al2O3 samples (Fig. 3b) indicate a change in the patterns due to nickel addition. The profile suggests a change from a mesoporous to a macroporous material and lowering of the surface areas (Table 1). The pore size distributions of the samples (Fig. 4) confirm the presence of
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Fig. 3. Nitrogen adsorption/desorption isotherms of samples heat-treated at different temperatures: (a) Al2O3 and (b) Ni/Al2O3. Closed symbols for adsorption, open symbols for desorption branch. Isotherms of samples heat-treated at 700 8C are vertically shifted 100 cm3/g for clarity.
micropores for the sample heat treated at 350 8C, but with an increase in the calcination temperature mesopore formation is significantly favored. This behaviour is attributed to carbon elimination at the higher treatment temperatures. With a heat treatment at 350 8C, the amount of residual material is still high. However, as summarized in Table 1, the higher surface area and the lower total pore volume of the sample heat treated at 350 8C may be due to the small pore size. The residual carbon elimination by heating at 550 and 700 8C resulted in a surface area decrease and thus, the residual carbon is thought to contribute significantly to a higher surface area. On the other hand, the carbon elimination promoted a great increase in the total pore volume, which suggests that the residual carbon present within the pores of the crystalline matrix is an amorphous carbon with high surface area. Burning off the residual carbon generated pores, which leads to mesopore formation and an increase in the total pore volume. The Ni doped Al2O3 sphere samples also presented a high surface area and pore volume. However, as shown in Table 1, these values were lower than those for the Al2O3 samples. The comparison of pore size distributions of Al2O3 and Ni/Al2O3 (Fig. 4a and b) shows clearly a difference in their pore sizes. The presence of nickel promoted the obtention of a sphere with meso- and Table 1 Chemical analysis and surface properties measured by N2 physisorption Samples
Tc# (8C)
Ni (wt.%)
S (m2 g1)
Vp (cm3 g1)
Al2O3 Al2O3 Al2O3 Ni/Al2O3 Ni/Al2O3
350 550 700 550 700
– – – 3.8 3.8
464 343 313 270 210
0.354 0.651 0.581 0.560 0.430
Tc#, temperature of calcination; S, specific surface area; Vp, pore volume.
macropores. Such a change in the pore size distribution cannot be attributed totally to the pore closing due to the NiO. As indicated by the hysteresis of the isotherms in Fig. 3b, the pore shapes of the Ni/Al2O3 samples appear to be slit-like. The presence of nickel promotes faster carbon elimination (as can be seen in the TG analyses) and such behaviour certainly affects the type of pores formed during the heat-treatment [24]. With the aim of identifying the phases present in the catalytic samples, X-ray diffraction and TPR analysis were carried out. Despite the NiAl2O4 and g-Al2O3 peaks being overlaid, in the XRD results of the freshly prepared catalysts presented in Fig. 5a the profiles show the NiAl2O4 formation, which is clearer for the sample heat treated at 700 8C. The XRD results also indicate the presence of the NiO phase. This suggests the possibility of obtaining catalytic sites with different properties after the activation. The shifts in the peaks at 2u = 458 and 668 are greater for the Ni/Al2O3 sample heat treated at 700 8C due to the presence of NiAl2O4, which means that the NiAl2O4 phase formation is greater for this sample. Fig. 5b shows the results of the XRD analysis of the reduced form of the catalysts. For the Ni/ Al2O3#550 catalyst there were more intense diffraction peaks related to the crystalline planes (1 1 1), (2 0 0) and (2 2 2) of the Ni, in comparison to the Ni/Al2O3#700 catalyst, indicating that the Ni/Al2O3#550 catalyst can be reduced more easily. In addition, a decrease in the intensity of the peaks corresponding to the crystalline planes of the Ni was observed for the Ni/ Al2O3#700 catalyst due to the presence of a strong metal-support interaction resulting from the high temperature of the heat treatment applied to the material. The determination of reducible species at the surface of the catalyst and the temperature at which these species are reduced, gives important information on catalysis. The H2 consumption peak near 530 8C and the shoulder near 630 8C in the TPR
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Fig. 4. Pore diameter distribution of the samples heat-treated at different temperatures: (a) Al2O3 and (b) Ni/Al2O3 calculated using the BJH model from the adsorption branch.
profile (Fig. 6) for the Ni/Al2O3 sample heat treated at 550 8C are due to the presence of NiO. For the Ni/Al2O3 sample heat treated at 700 8C, H2 consumption was observed at these temperatures but it was significantly lower, indicating that there is less NiO in this sample than in the sample heat treated at 550 8C. The Ni/Al2O3 sample heat treated at 700 8C gave a wide range of H2 consumption, beginning at 450 8C until the final temperature of the process. However, the maximum H2 consumption peak was observed at 830 8C, which is characteristic of the NiAl2O4 phase, indicating a greater metal-support interaction, promoted by the calcination temperature applied to the material. This observation is in agreement with the XRD profile, which indicates the presence of the NiO and NiAl2O4 phases [24]. Scanning electron microscopy (SEM) was carried out in order to observe the morphologies of the samples obtained. The SEM images presented in Fig. 7 shows the Al2O3 spheres heattreated at 550 8C. In spite of the low number of spheres presented in Fig. 7b, it is possible to estimate the mean size distribution (near 0.90 mm). The SEM image presented in Fig. 7a shows a pore diameter size quite different to that determined by the N2 adsorption isotherm, but it can illustrate the morphology, which is due to the utilization of organic compounds in the catalyst preparation. During the pyrolysis step the elimination of volatile materials occurs, which produces cavities as a result of their removal. At the same time, a solid rearrangement takes place, forming the crystalline matrix. 3.2. Catalytic testing
Typical experimental results obtained are presented in Table 2, in which the conversion of ethanol and the selectivities toward various reaction products are shown as a function of reaction temperature. It is observed that the conversion of ethanol reached values higher than 95%, for both catalysts (Ni/ Al2O3#550 and Ni/Al2O3#700) under the experimental conditions employed. For the Ni/Al2O3#700 catalyst at a reaction temperature of 450 8C, steam reforming of ethanol does not occur. Instead, dehydration of ethanol: C2 H5 OH ! C2 H4 þ H2 O
(4)
occurs to an appreciable extent producing only ethylene. This result is consistent with the characteristics of the support (gAl2O3), which has acidic sites that are required for the dehydration route [3,15,27]. In addition, according to the TPR and XRD results, the NiAl2O4 phase formation is greater for this sample, indicating a greater metal-support interaction. Thus, the active phase (Ni) is expected to be less accessible and the support mainly responsible for the catalytic activity. On the other hand, the Ni/Al2O3#550 catalyst, at the same reaction temperature, presented a distinct behaviour. The selectivities of H2, CH4, CO2, C2H4 and acetaldehyde (CH3CHO) were 61.5%, 0.5%, 1.4%, 34.6% and 2.0%, respectively, as shown in Table 2. From the analysis of the product distribution obtained, it appears that the predominant reactions are the reforming reaction of ethanol and water: C2 H5 OH þ H2 O ! CH4 þ CO2 þ 2H2 ;
(5)
ethanol dehydrogenation to acetaldehyde In order to investigate the catalytic activity of the Ni/Al2O3 spheres, the steam reforming of ethanol was carried out.
C2 H5 OH ! CH3 CHO þ H2
(6)
H.V. Fajardo, L.F.D. Probst / Applied Catalysis A: General 306 (2006) 134–141
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Fig. 6. Temperature programmed reduction (TPR) profiles of Ni/Al2O3 samples: (a) Ni/Al2O3 heat-treated at 550 8C and (b) Ni/Al2O3 heat-treated at 700 8C.
cleavage to produce H2, CH4 and CO2 [15,20]. Moreover, the amount of acetaldehyde detected could indicate that this particular catalyst has a mild capability for dehydrogenation of ethanol. Selectivity toward C2H4 formed by dehydration of ethanol over the acidic sites of the support, is relatively high and reaches a maximum at 450 8C. Ethylene production, however, decreases and selectivity toward H2 is favored as the temperature increases above 450 8C. This is the result of high rates of steam reforming of ethylene: C2 H4 þ 2H2 O ! CH4 þ CO2 þ 2H2 :
Fig. 5. (a) X-ray diffraction patterns for the samples after the pyrolysis step: (i) Al2O3 heat-treated at 550 8C, (ii) Al2O3 heat-treated at 700 8C, (iii) Ni/Al2O3 heat-treated at 550 8C and (iiii) Ni/Al2O3 heat-treated at 700 8C. (b) X-ray diffraction patterns for the catalysts in the reduced form: (iii0 ) Ni/Al2O3 heattreated at 550 8C and (iiii0 ) Ni/Al2O3 heat-treated at 700 8C.
and of course ethanol dehydration to ethylene. The Ni/ Al2O3#550 catalyst promotes ethanol reforming and H2 production significantly increases. This fact could be related to the more effective action of the active phase, which is more accessible in this sample. In the presence of Ni, the catalyst becomes more active and the active metal site may be responsible for the breaking of the C–C and C–H bonds in the ethanol
(7)
Selectivity toward acetaldehyde, which is formed by dehydrogenation of ethanol (relatively small amounts over Ni/Al2O3#550 catalyst), follows a qualitatively similar behaviour and is completely reformed at high temperatures: CH3 CHO þ H2 O ! CH4 þ CO2 þ 2H2 :
(8)
Apparently, these are intermediate reaction products, which undergo further reactions at higher temperatures [15]. During these runs a carbon imbalance was detected. For example, for the Ni/Al2O3#700 catalyst at a reaction temperature of 650 8C, the H2/carbon ratio was 3.8 and the maximum ratio H2/carbon in the steam reforming reaction is 3. These results suggest that the carbon balance is lower than 100% and filaments of carbon could be formed over the Ni surface and consequently the
Fig. 7. Scanning electron microscopy (SEM) images for the spheres heat-treated at 550 8C (a) and (b).
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Table 2 Conversion of ethanol and product selectivity values for Ni/Al2O3 catalysts (Ni/Al2O3#700 and Ni/Al2O3#550) at different steam reforming reaction temperatures Temperature (8C)
CEtOH (%)
SH2 (%)
SCH4 (%)
SCO2 (%)
SC2 H4 (%)
SCH3 CHO (%)
Ni/Al2O3#700 450 550 650
100 99.2 100
0 67.3 87.4
0 1.7 1.0
0 1.5 1.2
100 29.5 10.4
0 0 0
Ni/Al2O3#550 450 550 650
96.6 100 100
61.5 83.8 89.0
0.5 0.2 1.3
1.4 1.6 1.1
34.6 14.4 8.6
product in the reactor effluent is rich in H2 [28]. Moreover, after 5 h of operation, the reactor was plugged with carbon. However, the Raman spectra (Fig. 8) of the Ni/Al2O3#700 catalyst taken after the catalytic tests (at a reaction temperature of 650 8C) indicate only the presence of amorphous carbon. This may indicate that acetaldehyde and ethylene promote coke formation, as other authors have reported [1,15,29].
2.0 0 0
From the above results it is possible to propose a reaction scheme for ethanol steam reforming (Fig. 9) over Ni/Al2O3 spherical catalysts. Acetaldehyde and, principally, ethylene are intermediate products formed from ethanol dehydrogenation and dehydration, respectively. H2, CH4 and CO2 are final products obtained mainly through acetaldehyde and ethylene steam reforming which are favored through the excess of water in the system. 4. Conclusions
Fig. 8. Raman spectra of Ni/Al2O3#700 catalyst after the steam reforming of ethanol. Experimental conditions: mass of catalyst 100 mg, flow rate 30 mL min1, T = 650 8C.
In this study, a novel method of obtaining Al2O3 and Ni/ Al2O3 spheres was presented. It was observed that the methodology used in the preparation led to the obtention of materials with important properties for applications in catalytic processes, such as ethanol steam reforming. The results clarify that the novel Ni/Al2O3 catalysts produced a hydrogen-rich gas mixture in the temperature range of 450–650 8C and they show a high activity for ethanol steam reforming. The results obtained suggest that C2H4 and CH3CHO are intermediate products formed and they are converted into C1 compounds, so that, CH4, CO2 and H2 can be found in the outlet gas mixture. Since C2H4 is a precursor of coke formation and may lead to catalyst deactivation, its presence is undesirable. However, the formation of C2H4 as a product did not affect the overall H2 production. It is worth noting that, in all cases no CO was detected and Ni/Al2O3 spheres can be considered as a good choice for hydrogen production from ethanol steam reforming for fuel-cell applications. Acknowledgement The authors acknowledge the Brazilian funding support agency: CNPq. References
Fig. 9. Reaction mechanism scheme of ethanol steam reforming.
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Production of hydrogen by steam reforming of ethanol over Ni/Al2O3 spherical catalysts Humberto Vieira Fajardo *, Luiz Fernando Dias Probst Laborato´rio de Cata´lise Heterogeˆnea, Departamento de Quı´mica, Universidade Federal de Santa Catarina, 88040-900 Floriano´polis, SC, Brazil Received 25 November 2005; received in revised form 17 March 2006; accepted 20 March 2006 Available online 27 April 2006
Abstract A new and simple method to prepare Al2O3 and Ni/Al2O3 spherical catalysts for hydrogen production by steam reforming of ethanol is proposed. The present method was developed using a biopolymer (chitosan) and Al solution. A hybrid spherical compound of aluminum hydroxide and the biopolymer is formed. Through polymer elimination by thermal treatment, porous spheres with a high specific surface area and large pore volume are obtained. The Ni/Al2O3 spheres showed a high catalytic activity in ethanol steam reforming and hydrogen selectivity at temperatures between 450 and 650 8C. In all cases no CO was detected indicating that the Ni/Al2O3 spheres could be considered as a good choice for hydrogen production from ethanol steam reforming for fuel-cell applications. # 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Alumina; Catalysts; Steam reforming
1. Introduction Hydrogen is nowadays considered as an alternative fuel, and its production is a subject of current interest for fuel cell applications. Fuel cells are considered to have the potential to provide a clean energy source for automotive applications as an alternative to gasoline or diesel engines [1–4]. In recent years, the proton exchange membrane fuel cell (PEMFC) with hydrogen as the fuel has attracted great interest due to its potential application in electric vehicles and power stations. However, PEMFC uses pure H2 or H2-rich gas as a fuel, which needs to be stored or produced on-board a vehicle. Due to the lack of hydrogen storage and distribution infrastructure, the catalytic on-board production of H2-rich gas from a suitable fuel is attracting increasing attention [5–8]. Conventional methods for hydrogen production are based on gasoline, natural gas and methanol steam reforming [9]. However, although several sources of hydrogen have been investigated alcohols,
* Corresponding author at: Universidade Federal de Santa Catarina, Centro de Cieˆncias Fı´sicas e Matema´ticas, Departamento de Quı´mica, Po´s-Graduac¸a˜o em Quı´mica, Campus Universita´rio, CP 476, CEP 88040-900 Trindade, Floriano´polis, SC, Brazil. Tel.: +55 48 3331 9966; fax: +55 48 3331 9711. E-mail address: [email protected] (H.V. Fajardo). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.03.043
and in particular ethanol, which is a renewable material easily obtained from biomass, might be considered as a good choice, especially for Latin American countries with extensive plantations of sugar cane [9–11]. Ethanol has relatively high hydrogen content, and its reaction with water under steam reforming conditions: C2 H5 OH þ 3H2 O ! 2CO2 þ 6H2 ;
(1)
DH298 ¼ þ347:4 kJ=mol has been shown to be entirely feasible from a thermodynamic point of view [11–14]. In addition, the bio-ethanol-to-hydrogen system has the significant advantage of being nearly CO2 neutral, since the carbon dioxide produced is consumed for biomass growth, thus offering a nearly closed carbon loop [7,15]. In previous studies, several catalysts have been proposed to be further considered for practical applications in ethanol steam reforming for hydrogen production for fuel cell applications. Although many noble metals (Rh/Al2O3, Pt/Al2O3, Pd/ Al2O3, Ru/TiO2, Rh/CeO2–ZrO2) [3,16–19] and non-noble metals (Ni/Al2O3, Cu/Al2O3, Fe/Al2O3, Ni/La2O3, Ni/ La2O3–Al2O3, Ni/Y2O3, Co/Al2O3, Co/SiO2) [15,19–22] on different supports have been shown to be active in this process, Ni seems to be the preferred active ingredient because of its high activity and low cost.
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In this context the objective of this study was to develop a new, effective and highly efficient Ni-based catalyst for the steam reforming of ethanol to produce hydrogen with high selectivity. The synthesis method for the novel catalysts (Al2O3 and Ni-doped Al2O3) consists of obtaining a hybrid spherical compound of aluminum hydroxide and the organic polymer chitosan. Through the polymer elimination by thermal treatment, a porous Al2O3 sphere is obtained. The catalytic performances of the samples obtained were investigated in ethanol steam reforming. The key aim of this process is to maximize H2 production, discouraging at the same time reactions that lead to undesirable products, such as methane, acetaldehyde, diethyl ether or acetic acid, that compete with H2 for the hydrogen atoms, and minimize the formation of carbon monoxide, which acts as a poison to the fuel cell electrodes. In addition, the formation of large amounts of CO would require a complex gas clean up process downstream before feeding into the fuel cell [7]. The results of the catalytic behaviour of the Ni/ Al2O3 spheres in the steam reforming of ethanol for hydrogen production are reported here. 2. Experimental 2.1. Sample preparation and characterization For the Al2O3 spheres preparation, 1.50 g of chitosan (Aldrich) was dissolved in 50 mL of CH3COOH solution (5%, v/v) and 4.60 g of Al(NO3)39H2O (Riedel-de-Hae¨n) were dissolved in 20 mL of distilled water. The Al aqueous solution was then added to the polymer solution with stirring. The chitosan monomer to Al molar ratio was 1.5–2. The Al– chitosan solution was added into a NH4OH solution (50%, v/v) under vigorous stirring, in the form of drops with a syringe pump. The gel spheres formed were removed from the NH4OH solution and dried at ambient temperature for 72 h. The nickel addition (5 wt.%) was carried out by impregnation of an aqueous solution of Ni(NO3)26H2O (Fluka, 98%) on the Al2O3 spheres with stirring for 6 h. The samples with nickel were dried at ambient temperature for 24 h. The Al2O3 (Al2O3#350, Al2O3#550 and Al2O3#700) and Ni/Al2O3 (Ni/Al2O3#550 and Ni/Al2O3#700) samples were obtained by calcining the dried samples at 350, 550 and 700 8C in airflow for 1 h with a heating rate of 5 8C/min. The precursors were characterized by N2 adsorption/ desorption isotherms obtained at the temperature of liquid nitrogen in an automated physisorption instrument (Autosorb1C, Quantachrome Instruments). Prior to the measurements, the samples were outgassed in a vacuum at 200 8C for 2 h. Specific surface areas were calculated according to the Brunauer– Emmett–Teller (BET) method, and the pore size distributions were obtained according to the Barret–Joyner–Halenda (BJH) method from the adsorption data. Infrared patterns were obtained from 400 to 4000 cm1; the samples (chitosan and Al–chitosan dried samples, 2 mg) were mechanically blended with 200 mg of KBr. The data obtained were recorded using an FT Perkin-Elmer 16 PC infrared spectrophotometer.
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To monitor the pyrolysis process, a thermogravimetric analysis (TG) was performed with a Shimadzu TGA-50 thermobalance using 15 mg of sample with a heating rate of 10 8C/min under airflow of 50 mL/min. For the determination of the Ni content an atomic spectrometer (Varian Model SpectrAA 50), equipped with an air–acetylene flame atomizer and a Hitachi hollow cathode lamp (HLA—4S) was used. The instrument parameters were: wavelength 232 nm, slit width 1.0 nm, lamp current 4 mA, aspiration rate 5 mL/min and fuel acetylene with support air. The standard solutions for the Ni ion were prepared through appropriate dilution of a stock solution (Merck) containing 1000 ppm. Temperature programmed reduction (TPR) analysis was performed in a quartz reactor under 5 vol.% H2/N2 flow (30 mL/min) from 30 to 920 8C at a heating rate of 5 8C/min. A thermal conductivity detector (TCD) was used to monitor the H2 consumption. A PM5A column trapped water formed during the process. The crystalline phases (of freshly prepared catalysts and their reduced forms) were characterized by X-ray diffraction (XRD) in a Siemens D-5000, with graphite monochromated Cu Ka irradiation. The sample morphology was observed with scanning electron micrographs, obtained with a Philips XL30 scanning microscope operating at an accelerating voltage of 20 kV. The structures of the deposited carbon were characterized by means of Raman spectroscopy (Renishow Raman System 3000) at room temperature. Spectra were recorded using the 514.5 nm excitation line of an Ar ion laser. 2.2. Catalytic testing Catalytic performance tests were conducted at atmospheric pressure with a quartz fixed-bed reactor (inner diameter 12 mm) fitted in a programmable oven, in the temperature range of 450–650 8C. The catalyst was previously reduced in situ with hydrogen at 650 8C for 1 h. The water–ethanol mixture (molar ratio 3:1) was pumped into a heated chamber and vaporized. The water–ethanol gas (N2) stream (30 mL/ min) was then fed to the reactor containing 100 mg of catalyst. The reactants and the composition of the reactor effluent were analyzed with a gas chromatograph (Shimadzu GC 8A), equipped with a thermal conductivity detector (TCD), Porapak-Q and a 5A molecular sieve column with Ar as the carrier gas. It should be noted that no condensable products (such as acetic acid, diethyl ether and acetone) were detected in the reaction effluent under the analysis conditions employed. Reaction data were recorded for 5 h. Catalyst activity was evaluated in terms of ethanol conversion. We defined ethanol conversion as: CEtOH ð%Þ ¼
Qconv 100: QEtOH
(2)
Here, Qconv represents the quantity (moles) of converted ethanol; QEtOH represents the total quantity (moles) of ethanol fed into the reactor.
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We defined the catalyst selectivity as the mole fraction of each product as: SP ð%Þ ¼
QP 100: QsP
(3)
Here, QP represents the number of moles of each product; QsP represents the sum of the moles of the products, but the moles of solid products (such as small amounts of coke) are not included. It should be noted that the catalysts tested did not show deactivation during the reaction period (5 h of operation). 3. Results and discussion 3.1. Sample characterization One of the main applications of infrared spectra is in the identification of characteristic functional groups, where relatively large molecules are involved. The infrared spectra for chitosan (Cht) and the Al–chitosan composite (Al–Cht) taken before the calcination processes (Fig. 1) were analyzed to obtain information about the functional groups that participate in the binding or interaction with Al in the intermediate stage of the porous Al2O3 spheres synthesis. The hydroxyl groups of the free alcohols, thus not associated, have O–H stretching in the form of a narrow band at 3620 cm1. In polymeric associations hydroxyl groups absorb in the form of a broad band at around 3400 cm1 [23]. The bands in the region of 3440 cm1 in the two spectra are associated with stretching of the OH groups of the chitosan biopolymer. There was no displacement in relation to the wavenumber indicating that the intermolecular interactions by way of hydrogen bonds of the biopolymer were maintained. The decrease in the intensity of the band on the spectrum for the compound (Al–chitosan) is due to the interaction of Al with the oxygen atoms of the hydroxyl groups of the biopolymer bound to the glycoside ring. The Al ions are preferentially bound to oxygen. The disappearance of the band at around 1650 cm1 is related to the interaction of Al with carbonyl groups from the partially deacetylated chitin residues.
Fig. 1. Infrared spectra of chitosan (Cht) and chitosan–Al (Al–Cht) composite before the heat-treatment process. Inset: chemical structure of chitosan.
Fig. 2. Thermogravimetric analysis of chitosan–Ni–Al (Cht–Ni–Al) and chitosan–Al (Cht–Al).
In the region around 1100 cm1 there was a decrease in the characteristic stretching of the C–O groups of the carbon bound to the OH group (primary alcohol) of chitosan due to the interaction with Al. The infrared spectra therefore indicate a modification in certain characteristic regions of the functional groups of the biopolymer susceptible to interaction with Al, without there being a significant modification in the semi crystalline chitosan structure. The thermogravimetric analysis for the samples presented in Fig. 2, shows the elimination of residue from the starting material, which depends on the sample composition (chitosan– Al and chitosan–Ni–Al). The nickel presence promotes faster carbon elimination during the heat-treatment. The residue elimination is observed in different temperature ranges with a substantial weight loss, near 75 wt.% for both samples. A clear inflexion point near 250 8C is mainly attributed to the external organic material elimination, which is easily burned off together with the residual nitrate. The chitosan–Ni–Al (Cht– Ni–Al) exhibited a second inflexion point located at 380 8C, while for chitosan–Al (Cht–Al) this second inflexion point was at 310 8C with another at 530 8C, being attributed to the residual carbon present within the spheres. This distinct behaviour is due to the presence of nickel. It is known that a metal (Ni) load in the sample promotes the elimination of carbonaceous materials at a lower temperature [24–26]. The TGA profile suggests a temperature of 400 and 600 8C for the total elimination of the residual material with a short calcination time for the chitosan–Ni–Al and chitosan–Al samples, respectively. The samples were heat-treated at 350, 550 and 700 8C under airflow for 1 h. The N2 adsorption/desorption isotherms of the Al2O3 samples heat-treated at 550 and 700 8C show type-IV (IUPAC) curves, pointing to the mesoporous material. By contrast, the Al2O3 sample heat-treated at 350 8C shows a microporous isotherm (Fig. 3a). The N2 adsorption/desorption isotherms of the Ni/Al2O3 samples (Fig. 3b) indicate a change in the patterns due to nickel addition. The profile suggests a change from a mesoporous to a macroporous material and lowering of the surface areas (Table 1). The pore size distributions of the samples (Fig. 4) confirm the presence of
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Fig. 3. Nitrogen adsorption/desorption isotherms of samples heat-treated at different temperatures: (a) Al2O3 and (b) Ni/Al2O3. Closed symbols for adsorption, open symbols for desorption branch. Isotherms of samples heat-treated at 700 8C are vertically shifted 100 cm3/g for clarity.
micropores for the sample heat treated at 350 8C, but with an increase in the calcination temperature mesopore formation is significantly favored. This behaviour is attributed to carbon elimination at the higher treatment temperatures. With a heat treatment at 350 8C, the amount of residual material is still high. However, as summarized in Table 1, the higher surface area and the lower total pore volume of the sample heat treated at 350 8C may be due to the small pore size. The residual carbon elimination by heating at 550 and 700 8C resulted in a surface area decrease and thus, the residual carbon is thought to contribute significantly to a higher surface area. On the other hand, the carbon elimination promoted a great increase in the total pore volume, which suggests that the residual carbon present within the pores of the crystalline matrix is an amorphous carbon with high surface area. Burning off the residual carbon generated pores, which leads to mesopore formation and an increase in the total pore volume. The Ni doped Al2O3 sphere samples also presented a high surface area and pore volume. However, as shown in Table 1, these values were lower than those for the Al2O3 samples. The comparison of pore size distributions of Al2O3 and Ni/Al2O3 (Fig. 4a and b) shows clearly a difference in their pore sizes. The presence of nickel promoted the obtention of a sphere with meso- and Table 1 Chemical analysis and surface properties measured by N2 physisorption Samples
Tc# (8C)
Ni (wt.%)
S (m2 g1)
Vp (cm3 g1)
Al2O3 Al2O3 Al2O3 Ni/Al2O3 Ni/Al2O3
350 550 700 550 700
– – – 3.8 3.8
464 343 313 270 210
0.354 0.651 0.581 0.560 0.430
Tc#, temperature of calcination; S, specific surface area; Vp, pore volume.
macropores. Such a change in the pore size distribution cannot be attributed totally to the pore closing due to the NiO. As indicated by the hysteresis of the isotherms in Fig. 3b, the pore shapes of the Ni/Al2O3 samples appear to be slit-like. The presence of nickel promotes faster carbon elimination (as can be seen in the TG analyses) and such behaviour certainly affects the type of pores formed during the heat-treatment [24]. With the aim of identifying the phases present in the catalytic samples, X-ray diffraction and TPR analysis were carried out. Despite the NiAl2O4 and g-Al2O3 peaks being overlaid, in the XRD results of the freshly prepared catalysts presented in Fig. 5a the profiles show the NiAl2O4 formation, which is clearer for the sample heat treated at 700 8C. The XRD results also indicate the presence of the NiO phase. This suggests the possibility of obtaining catalytic sites with different properties after the activation. The shifts in the peaks at 2u = 458 and 668 are greater for the Ni/Al2O3 sample heat treated at 700 8C due to the presence of NiAl2O4, which means that the NiAl2O4 phase formation is greater for this sample. Fig. 5b shows the results of the XRD analysis of the reduced form of the catalysts. For the Ni/ Al2O3#550 catalyst there were more intense diffraction peaks related to the crystalline planes (1 1 1), (2 0 0) and (2 2 2) of the Ni, in comparison to the Ni/Al2O3#700 catalyst, indicating that the Ni/Al2O3#550 catalyst can be reduced more easily. In addition, a decrease in the intensity of the peaks corresponding to the crystalline planes of the Ni was observed for the Ni/ Al2O3#700 catalyst due to the presence of a strong metal-support interaction resulting from the high temperature of the heat treatment applied to the material. The determination of reducible species at the surface of the catalyst and the temperature at which these species are reduced, gives important information on catalysis. The H2 consumption peak near 530 8C and the shoulder near 630 8C in the TPR
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Fig. 4. Pore diameter distribution of the samples heat-treated at different temperatures: (a) Al2O3 and (b) Ni/Al2O3 calculated using the BJH model from the adsorption branch.
profile (Fig. 6) for the Ni/Al2O3 sample heat treated at 550 8C are due to the presence of NiO. For the Ni/Al2O3 sample heat treated at 700 8C, H2 consumption was observed at these temperatures but it was significantly lower, indicating that there is less NiO in this sample than in the sample heat treated at 550 8C. The Ni/Al2O3 sample heat treated at 700 8C gave a wide range of H2 consumption, beginning at 450 8C until the final temperature of the process. However, the maximum H2 consumption peak was observed at 830 8C, which is characteristic of the NiAl2O4 phase, indicating a greater metal-support interaction, promoted by the calcination temperature applied to the material. This observation is in agreement with the XRD profile, which indicates the presence of the NiO and NiAl2O4 phases [24]. Scanning electron microscopy (SEM) was carried out in order to observe the morphologies of the samples obtained. The SEM images presented in Fig. 7 shows the Al2O3 spheres heattreated at 550 8C. In spite of the low number of spheres presented in Fig. 7b, it is possible to estimate the mean size distribution (near 0.90 mm). The SEM image presented in Fig. 7a shows a pore diameter size quite different to that determined by the N2 adsorption isotherm, but it can illustrate the morphology, which is due to the utilization of organic compounds in the catalyst preparation. During the pyrolysis step the elimination of volatile materials occurs, which produces cavities as a result of their removal. At the same time, a solid rearrangement takes place, forming the crystalline matrix. 3.2. Catalytic testing
Typical experimental results obtained are presented in Table 2, in which the conversion of ethanol and the selectivities toward various reaction products are shown as a function of reaction temperature. It is observed that the conversion of ethanol reached values higher than 95%, for both catalysts (Ni/ Al2O3#550 and Ni/Al2O3#700) under the experimental conditions employed. For the Ni/Al2O3#700 catalyst at a reaction temperature of 450 8C, steam reforming of ethanol does not occur. Instead, dehydration of ethanol: C2 H5 OH ! C2 H4 þ H2 O
(4)
occurs to an appreciable extent producing only ethylene. This result is consistent with the characteristics of the support (gAl2O3), which has acidic sites that are required for the dehydration route [3,15,27]. In addition, according to the TPR and XRD results, the NiAl2O4 phase formation is greater for this sample, indicating a greater metal-support interaction. Thus, the active phase (Ni) is expected to be less accessible and the support mainly responsible for the catalytic activity. On the other hand, the Ni/Al2O3#550 catalyst, at the same reaction temperature, presented a distinct behaviour. The selectivities of H2, CH4, CO2, C2H4 and acetaldehyde (CH3CHO) were 61.5%, 0.5%, 1.4%, 34.6% and 2.0%, respectively, as shown in Table 2. From the analysis of the product distribution obtained, it appears that the predominant reactions are the reforming reaction of ethanol and water: C2 H5 OH þ H2 O ! CH4 þ CO2 þ 2H2 ;
(5)
ethanol dehydrogenation to acetaldehyde In order to investigate the catalytic activity of the Ni/Al2O3 spheres, the steam reforming of ethanol was carried out.
C2 H5 OH ! CH3 CHO þ H2
(6)
H.V. Fajardo, L.F.D. Probst / Applied Catalysis A: General 306 (2006) 134–141
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Fig. 6. Temperature programmed reduction (TPR) profiles of Ni/Al2O3 samples: (a) Ni/Al2O3 heat-treated at 550 8C and (b) Ni/Al2O3 heat-treated at 700 8C.
cleavage to produce H2, CH4 and CO2 [15,20]. Moreover, the amount of acetaldehyde detected could indicate that this particular catalyst has a mild capability for dehydrogenation of ethanol. Selectivity toward C2H4 formed by dehydration of ethanol over the acidic sites of the support, is relatively high and reaches a maximum at 450 8C. Ethylene production, however, decreases and selectivity toward H2 is favored as the temperature increases above 450 8C. This is the result of high rates of steam reforming of ethylene: C2 H4 þ 2H2 O ! CH4 þ CO2 þ 2H2 :
Fig. 5. (a) X-ray diffraction patterns for the samples after the pyrolysis step: (i) Al2O3 heat-treated at 550 8C, (ii) Al2O3 heat-treated at 700 8C, (iii) Ni/Al2O3 heat-treated at 550 8C and (iiii) Ni/Al2O3 heat-treated at 700 8C. (b) X-ray diffraction patterns for the catalysts in the reduced form: (iii0 ) Ni/Al2O3 heattreated at 550 8C and (iiii0 ) Ni/Al2O3 heat-treated at 700 8C.
and of course ethanol dehydration to ethylene. The Ni/ Al2O3#550 catalyst promotes ethanol reforming and H2 production significantly increases. This fact could be related to the more effective action of the active phase, which is more accessible in this sample. In the presence of Ni, the catalyst becomes more active and the active metal site may be responsible for the breaking of the C–C and C–H bonds in the ethanol
(7)
Selectivity toward acetaldehyde, which is formed by dehydrogenation of ethanol (relatively small amounts over Ni/Al2O3#550 catalyst), follows a qualitatively similar behaviour and is completely reformed at high temperatures: CH3 CHO þ H2 O ! CH4 þ CO2 þ 2H2 :
(8)
Apparently, these are intermediate reaction products, which undergo further reactions at higher temperatures [15]. During these runs a carbon imbalance was detected. For example, for the Ni/Al2O3#700 catalyst at a reaction temperature of 650 8C, the H2/carbon ratio was 3.8 and the maximum ratio H2/carbon in the steam reforming reaction is 3. These results suggest that the carbon balance is lower than 100% and filaments of carbon could be formed over the Ni surface and consequently the
Fig. 7. Scanning electron microscopy (SEM) images for the spheres heat-treated at 550 8C (a) and (b).
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Table 2 Conversion of ethanol and product selectivity values for Ni/Al2O3 catalysts (Ni/Al2O3#700 and Ni/Al2O3#550) at different steam reforming reaction temperatures Temperature (8C)
CEtOH (%)
SH2 (%)
SCH4 (%)
SCO2 (%)
SC2 H4 (%)
SCH3 CHO (%)
Ni/Al2O3#700 450 550 650
100 99.2 100
0 67.3 87.4
0 1.7 1.0
0 1.5 1.2
100 29.5 10.4
0 0 0
Ni/Al2O3#550 450 550 650
96.6 100 100
61.5 83.8 89.0
0.5 0.2 1.3
1.4 1.6 1.1
34.6 14.4 8.6
product in the reactor effluent is rich in H2 [28]. Moreover, after 5 h of operation, the reactor was plugged with carbon. However, the Raman spectra (Fig. 8) of the Ni/Al2O3#700 catalyst taken after the catalytic tests (at a reaction temperature of 650 8C) indicate only the presence of amorphous carbon. This may indicate that acetaldehyde and ethylene promote coke formation, as other authors have reported [1,15,29].
2.0 0 0
From the above results it is possible to propose a reaction scheme for ethanol steam reforming (Fig. 9) over Ni/Al2O3 spherical catalysts. Acetaldehyde and, principally, ethylene are intermediate products formed from ethanol dehydrogenation and dehydration, respectively. H2, CH4 and CO2 are final products obtained mainly through acetaldehyde and ethylene steam reforming which are favored through the excess of water in the system. 4. Conclusions
Fig. 8. Raman spectra of Ni/Al2O3#700 catalyst after the steam reforming of ethanol. Experimental conditions: mass of catalyst 100 mg, flow rate 30 mL min1, T = 650 8C.
In this study, a novel method of obtaining Al2O3 and Ni/ Al2O3 spheres was presented. It was observed that the methodology used in the preparation led to the obtention of materials with important properties for applications in catalytic processes, such as ethanol steam reforming. The results clarify that the novel Ni/Al2O3 catalysts produced a hydrogen-rich gas mixture in the temperature range of 450–650 8C and they show a high activity for ethanol steam reforming. The results obtained suggest that C2H4 and CH3CHO are intermediate products formed and they are converted into C1 compounds, so that, CH4, CO2 and H2 can be found in the outlet gas mixture. Since C2H4 is a precursor of coke formation and may lead to catalyst deactivation, its presence is undesirable. However, the formation of C2H4 as a product did not affect the overall H2 production. It is worth noting that, in all cases no CO was detected and Ni/Al2O3 spheres can be considered as a good choice for hydrogen production from ethanol steam reforming for fuel-cell applications. Acknowledgement The authors acknowledge the Brazilian funding support agency: CNPq. References
Fig. 9. Reaction mechanism scheme of ethanol steam reforming.
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